Chemical mechanical polishing, also known as chemical mechanical planarization (referred to herein collectively as “CMP”), has been widely utilized for the planarization of semiconductor wafers. CMP produces a substantially smooth, planar face on a major surface of the wafer (referred to herein as the wafer's front surface) to prepare the wafer for subsequent fabrication processes (e.g., photo-resist coating, pattern definition, etc.). During CMP, an unprocessed wafer is typically first transferred to a wafer carrier head, which presses the wafer against a polish pad (or other polishing surface) supported by a platen. Polishing slurry is introduced between the wafer's front surface and the polish pad, and relative motion (e.g., rotational, orbital, and/or linear) is initiated between the polish pad and the carrier head. The mechanical abrasion of the polish pad and the chemical interaction of the slurry gradually remove topographical irregularities present on the wafer's front surface to produce a planar surface.
One known type of carrier head comprises a housing having a flexible bladder coupled thereto, which contacts the back (i.e., the unpolished) surface of the wafer during polishing. The housing and the bladder cooperate to form a plurality of concentric pressure chambers or plenums behind the bladder. During CMP, the pressure within each of these plenums is independently adjusted to vary the force applied to the wafer's back surface by the bladder at different annular zones and consequently control the rate removal at different annular zones along the wafer's front surface. In this manner, the carrier head may compensate for topographical variations on wafer's front surface. For example, if a particular portion of wafer's front surface is determined to be relatively thick, the pressure within the corresponding plenum may be increased to intensify the rate of removal proximate the thicker area. Plenum pressure adjustments are typically performed by a closed-loop control (CLC) system, which may comprise a central controller and a thickness measuring system.
One known type of thickness measuring system is an induction system. An induction system employs one or more eddy current probes, which may be fixedly disposed within the polishing platen at different radial positions. As the platen moves relative to the wafer, the eddy current probes gather wafer thickness readings. The controller compiles the wafer thickness readings to produce a topographical wafer map, which the controller then utilizes to determine appropriate plenum pressure adjustments.
Another known type of thickness measuring system is an optical probe system. An optical probe system employs one or more optical probes which may be fixedly disposed within the polishing platen at different radial positions. As the platen moves relative to the wafer, the optical probes gather wafer thickness readings. The controller compiles the wafer thickness readings to produce a topographical wafer map, which the controller then utilizes to determine appropriate plenum pressure adjustments. The optical probes may employ specific wavelengths of light such as the visible spectrum, infrared, or ultraviolet.
To accurately compile the wafer thickness data received from the thickness-sensing probes, the controller must estimate probe radial position for each wafer thickness measurement. Generally, probe location is determined by reference to a metallic (e.g., copper) film deposited on the wafer's front surface during patterning. The outer edge of the film and the outer edge of the wafer are typically separated by an annular gap, which is commonly referred to as edge exclusion. This annular gap has a predetermined width or edge exclusion value, which may be, for example, 3 millimeters for a 300 millimeter wafer. Conventional probe location methods assume a constant edge exclusion value in estimating probe radial position. That is, such methods estimate a probe's radial position to be the edge exclusion value (e.g., 3 millimeters) away for the outer edge of the wafer at the moment the probe sweeps across the film's outer edge.
In practice, edge exclusion often does not remain constant during wafer processing. Instead, the outer edge of the metallic film tends to recede inward. This phenomenon of “edge burn” results in an increase in edge exclusion. In certain instances, edge burn may approach or even exceed 10 millimeters. Due to this change in edge exclusion, conventional probe location methods often inaccurately estimate the true radial positions of the probes and thus assign incorrect probe radial positions to the wafer thickness measurements. As a result, such probe location methods are prone to produce inaccurate wafer maps, which lead to improper plenum pressure adjustments, a less precise polishing process, and, ultimately, a lower die yield.
In view of the above, it should be appreciated that it would be desirable to provide a method for monitoring the change in edge exclusion during CMP processing that allows for a more accurate estimation of probe radial position. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.